fix: ProtoBuf evidence not repeated, but optional (#425)

fixes #422 
and yes, fixing this bug is actually considered a breaking change in
terms of ProtoBuf

Author: stevespringett

schema/bom-1.6.proto

@@ -134,7 +134,7 @@ message Component {
   // Specifies optional, custom, properties
   repeated Property properties = 22;
   // Specifies optional license and copyright evidence
-  repeated Evidence evidence = 23;
+  optional Evidence evidence = 23;
   // Specifies optional release notes.
   optional ReleaseNotes releaseNotes = 24;
   // A model card describes the intended uses of a machine learning model, potential limitations, biases, ethical considerations, training parameters, datasets used to train the model, performance metrics, and other relevant data useful for ML transparency.